(1) Field of the Invention
The present invention relates to an image forming device that includes a cleaning unit for cleaning the surface of an image carrier by contacting with the surface, and to an image forming method for the image forming device.
(2) Description of the Related Art
In recent years, tandem printers have become prevalent. In these tandem printers, a plurality of image creating units, a photosensitive drum, and a transferring unit are arranged along an intermediate transfer belt, toner images formed by the image creating units are transferred onto the intermediate transfer belt by a multiple transfer, and the toner images of respective colors are transferred from the intermediate transfer belt onto a recording sheet all at once to obtain a full-color image.
In such printers, a cleaner is used to remove remnant toners remaining on the surface of the intermediate transfer belt after the transferring, or to remove paper powder or the like attached to the surface of the transfer belt. A typical method of cleaning the remnant toners or the like is to press a cleaning blade, which is made of an elastic material such as polyurethane, onto the intermediate transfer belt surface to collect the remnant toners or the like by shaving them off.
In this method of contacting the cleaning blade, however, as shown in FIG. 16A, as the number of prints increases, more amounts of remnant toner, paper powder, dust and the like become present between a contacting surface 903 of a cleaning blade 901 and an intermediate transfer belt 902. Then it becomes as if a foreign object is inserted between the cleaning blade 901 and the intermediate transfer belt 902, in which minute gaps are created between them and toners and the like pass through the gaps to cause a defective cleaning.
Japanese Patent Application Publication No. 2005-300916 discloses a construction in which the intermediate transfer belt is rotated in the reverse direction by a predetermined amount before the belt is driven to be rotated in the positive direction (positive rotation). With this reverse rotation, the paper powder and the like are liberated from between the cleaning blade 901 and the intermediate transfer belt 902, and fly away from the cleaning blade 901, as shown in FIG. 16B. This allows the cleaning blade 901 and the intermediate transfer belt 902 to return to the normal contact state in which the foreign object has been removed from between the cleaning blade 901 and the intermediate transfer belt 902, as shown in FIG. 16C.
FIG. 16C shows the state in which the cleaning blade 901 is in close contact with the intermediate transfer belt 902. In the actuality, however, when the cleaning blade 901 and the intermediate transfer belt 902 returns to the normal contact state as the positive rotation is started, remnants such as the toner and a toner additive like silica remaining between the cleaning blade 901 and the intermediate transfer belt 902 play a role of a lubricant agent that keeps the frictional force, which occurs between the cleaning blade 901 and the intermediate transfer belt 902, to an appropriate size. And this prevents an inverse warpage of the cleaning blade 901 which occurs due to the friction.
The paper powder and the like having flown away come back before the cleaning blade 901 as the intermediate transfer belt 902 rotates in the positive direction, and are shaved off and collected by the cleaning blade 901 since the contact state of the cleaning blade 901 has returned to the normal state by then.
However, with the construction disclosed in the above-mentioned Japanese Patent Application Publication, an inverse warpage may occur, for the following reasons.
For example, when an image read out from a document having a low coverage rate is printed continuously onto a large number of sheets, only a small amount of toner is transferred to the surface of the intermediate transfer belt 902, and thus a small amount of toner and toner additive becomes present between the cleaning blade 901 and the intermediate transfer belt 902.
If, in this state after the print job, the intermediate transfer belt 902 is driven to be rotated in the reverse and positive directions before it is rotated in the positive direction for another job, the toner and toner additive, which are present between the cleaning blade 901 and the intermediate transfer belt 902 and play a role of a lubricant agent although a small amount, are removed from the cleaning blade 901 by the reverse rotation, and hardly exist when the intermediate transfer belt 902 is rotated in the positive direction.
This increase the frictional force between the cleaning blade 901 and the intermediate transfer belt 902. And when this happens, the edge of the cleaning blade 901 is pulled by the intermediate transfer belt 902 that moves in the positive direction, and is warped in the inverse direction by the force of the moving intermediate transfer belt 902, as shown in FIG. 16D.
When such an inverse warpage occurs, the cleaning blade 901 may be deformed or cut in part, becomes unable to shave the toner off fully, and the cleaning performance is degraded.
This problem is not limited to the cleaning blade for cleaning the intermediate transfer belt, but may also occur, for example, to the cleaning blade for cleaning remnant toner from the photosensitive drum.